What Fire Does to Structural Wood vs. Steel vs. Concrete

Wood is combustible, so fire burns away the outer layers of beams and joists, leaving a charred surface. The remaining unburned wood may still carry load, but the loss of cross-section reduces its strength. The rate of charring depends on the wood species, moisture content, and fire intensity, so the damage is not uniform across a structure.

After a fire, charred wood can also absorb moisture from firefighting efforts, leading to further decay and mold growth. A structural engineer can measure the remaining sound wood and determine if a member can be reinforced or must be replaced. In regions with humid summers and cold winters, trapped moisture in charred wood is a real concern for long-term damage.

Concrete is inherently fire-resistant because it does not burn and conducts heat slowly. However, rapid heating and cooling can cause spalling, where the surface flakes off, exposing the aggregate underneath. This is especially common if the concrete had high moisture content, as steam pressure builds inside and forces the surface to pop.

Reinforced concrete contains steel rebar, which can lose strength if heated enough. The concrete's slower heat transfer usually protects the rebar, but in a severe fire, the rebar can be compromised. Cracks from thermal expansion can also allow water to enter, leading to further deterioration over time.

A fire-damaged concrete slab might look fine on the surface but have hidden internal cracks. A professional can perform non-destructive testing to assess the concrete's integrity. If spalling is extensive, the damaged layer may need to be removed and repaired.

• Look for surface spalling or flaking concrete.
• Check for cracks radiating from corners or around openings.
• Tap the surface with a hammer to detect hollow sounds.
• Examine exposed rebar for rust or heat damage.
• Note any areas where the concrete has changed color.

First, make sure the fire department has declared the structure safe to enter. Then, document the damage with photos and notes for your insurance company. Do not attempt to clean or repair anything until a professional has assessed the structural safety, as moving debris or removing charred materials can make the situation worse.

Contact your insurance company to understand what is covered and what documentation they need. Policies vary, and your insurer decides what is covered. Then, arrange for a structural inspection to get a clear picture of the damage. The pros can help you develop a plan for drying, repairing, and restoring your home.

• Wait for the fire department to clear the building.
• Take photos of all damage, including close-ups of charred wood and spalled concrete.
• Keep a list of damaged items for your insurance claim.
• Avoid entering rooms with sagging ceilings or floors.
• Turn off utilities only if it is safe to access them.
• Call a structural engineer before any cleanup begins.